An error correction and detection system is important in computer science and information systems, and especially applicable in communications systems because an error correction and detection system detects errors due to noise or other signal impairments that result during signal transmission. Error correction enables error localization and corrects errors.
In telecommunications, forward error correction (FEC) allows error control for data transmission, and differs from standard error detection and correction because the receiver can correct some errors without requesting a retransmission of data. The design of the code used in the FEC system determines the maximum fraction of errors that can be corrected in advance. As a result, different FEC codes can be suitable for different signal and transmission conditions.
In many error correcting and detection systems such as FEC, redundancy is added to transmitted information using a predetermined algorithm. Each of the redundant bits is a complex function of the original information bits. As a result, the encoded output may or may not include the original information. For example, unmodified inputs at the encoder output are systematic codes and those that are not are non-systematic.
FEC can be considered as an error system that averages the noise because each input data bit affects many transmitted symbols. Some symbols are more corrupted by noise than others and this allows the original data to be extracted from the other, less corrupted received signals that depend on the same user input data.
Block coding and convolutional coding are two major FEC systems used in many commercially available communications systems. Block codes use fixed-size blocks or packets of bits or symbols of predetermined size, while a convolutional code, on the other hand, works on bit and symbol streams of arbitrary length. Convolutional codes are usually decoded using the maximum likelihood sequence estimator algorithm, also known as the Viterbi algorithm or the Viterbi decoder.
In some applications, it is desirable to add different levels of protection to different portions of the data packet or transmission. For example, headers in a packet contain important data, often considered more important than the remaining data contained in the packet. In some cases, it is desirable if the headers can be received with higher reliability than the data or digital voice contained in the balance of the packet or transmission. The headers can contain destination data, identifiers, a cyclic redundancy check, and other bit information and data that is important to protect. Some prior art proposals encode/decode and interleave/deinterleave separately the headers, digital voice and data. This has some disadvantages since each FEC process for the separate data pieces requires flush bits and smaller interleavers.